The present invention relates generally to optical fiber amplifiers. More specifically, the present invention relates to a network of fiber amplifiers having a coherently combined output. Advantageously, high-power laser amplifiers can be fabricated by employing a plurality of these fiber amplifier networks.
In numerous applications such as laser tracking, laser guidance and laser imaging, it is desirable to produce a high power coherent output laser beam. Moreover, high power coherent laser systems find applications in such diverse fields as offensive and defensive weapon systems, e.g., non-visible light illuminators for special operation forces and protective laser grids, as well as material processing, e.g., welding, cutting, heat treating and ablating, and medicine, e.g., surgical and diagnostic aides. In short, there are multiple military and commercial applications for high power fundamental mode laser sources. Fiber optic amplifiers have generated single mode signal powers in the 10-20 watt range; fiber optic amplifiers have not yet demonstrated output signal power in the 50-100 watts (or greater) range.
In the earliest laser systems, single semiconductor lasers were utilized to provide a coherent source of laser output. These single semiconductor lasers were limited in the amount of power which they could provide due to their structural limitations and limited efficiency. Subsequently, arrays of semiconductor lasers have been utilized in which adjacent emitters of the array of semiconductor lasers spaced upon the same substrate are coupled together. One such laser array system was disclosed in commonly assigned U.S. Pat. No. 5,212,707 to Heidel et al., which patent is incorporated herein by reference for all purposes.
A two-dimensional semiconductor laser array can be fabricated from a plurality of the one-dimensional semiconductor laser arrays by the simple expedient of stacking the one-dimensional semiconductor laser arrays within a heatsink which serves as a holding or clamping fixture. It will be appreciated that the clamping fixture can be designed such that the one-dimensional semiconductor laser arrays may be stacked on top of one another so that the outputs of each one-dimensional semiconductor laser array are substantially parallel to the outputs of the other semiconductor laser arrays. This two-dimensional laser array, when properly supplied with power, produces a single collimated spot of laser output in the far field. By utilizing a plurality of one-dimensional semiconductor laser arrays whose outputs may be combined, the output power of the two-dimensional semiconductor laser array may be quite high. For example, 25 watts of continuous wave laser energy was produced by a two-dimensional semiconductor laser array consisting of twelve one-dimensional semiconductor laser arrays with each one-dimensional semiconductor laser array having twenty-one emitters. Additionally, the overall efficiency of the laser array from electrical input to power in the central lobe was approximately 26%.
U.S. Pat. No. 5,299,222 discloses an alternative approach to producing a high power laser diode system that collects and concentrates laser output from a stack of diode laser bars in a form that is most useful for pumping a laser, e.g., a solid state laser. As described in U.S. Pat. No. 5,299,222, the light beam output of stacked diode laser bars is coupled into a plurality of optical fibers. The output light beams from the fibers may be used to pump a laser resonator. The fibers can be grouped at various end points of a solid-state laser cavity for efficient end-pumping. It will be appreciated that expansion of the systems discussed immediately above would require both a large amount of real estate and complex optic assemblies to couple the outputs of a plurality of the disclosed output modules to a single spot.
More recently, fiber optic power amplifiers have been employed to produce a high-power output signal. A single fiber power amplifier will suffice for some low power applications. However, a coherent array of optical fiber amplifiers collectively forming the fiber optic power amplifier can be employed in those specific applications when higher power output laser beams are required. One example of a coherent phased array of fiber optic amplifiers suitable for use in the present invention for generating high-power laser beams needed for long range ladar system applications is shown in FIG. 4. This particular laser power amplifier is described in detail in copending, commonly assigned U.S. patent application Ser. Nos. 08/471,870 and 08/611,474, which applications are incorporated herein by reference for all purposes. It will be appreciated that the power splitter, amplifier and phase modulator elements in FIG. 4 may be arranged in various configurations other than the exemplary arrangement illustrated in that Figure.
The fiber optic power amplifier 600 illustrated in FIG. 4 includes a first stage composed of a first beam splitter element 610, for splitting a received laser beam into a number N of secondary laser beams. Each of the secondary laser beams is provided to a second beam splitter element 620, which produces a number M of tertiary laser beams from a respective one of the secondary laser beams. Each of the tertiary laser beams is amplified by a respective fiber amplifier generally denoted 630. It will be appreciated that although two separate stages of beam splitter elements 610, 620 and one amplifier stage 630 are depicted in FIG. 4, the fiber optic power amplifier 600 can have more of less amplification stages. For example, when the first and second beam splitter elements 610, 620 include an optical amplifier 16 pumped by a pump source 18, a beam splitter 24 and, optionally, a number N.times.M phase modulators, respectively, a total of three amplification devices are included in the power amplifier 600. See FIG. 5a.
Alternative configurations are also possible. For example, the number of series connected elements, i.e., 610, 620 can be any number greater than or equal to 2. Moreover, it should be mentioned that the beam splitter construction is not limited to the arrangement illustrated in FIG. 5a. For example, the first stage element 610 need not include either an amplifier 16 or a phase modulator 27 (Fig. 5b); alternatively, the first stage element 610 may include optical amplifier 16 but omit phase modulator 27 (FIG. 5c). Needless to say, additional amplifier stages can be provided.
It will be noted that the fiber optic power amplifier 600 includes a phase modulator 27 in each optical path terminating at an output device. It will be appreciated that the phase modulators are provided to ensure that all of the N.times.M laser beams output by power amplifier 600 arrive at the output device, e.g., a lens, with a predetermined phase profile to minimize the losses produced in output device. The power amplifier 600 of FIG. 4 includes a waveform sensor 640 in the output optical path, wherein the sensor signals are provided to phase modulators 27 in elements 610, 620 via an adaptive waveform controller 650. Examples of the construction and operation of waveform sensor 640 and waveform controller 650 are provided in above-referenced copending, commonly assigned U.S. patent application Ser. Nos. 08/471,870 and 08/611,474.
Although not explicitly illustrated in FIGS. 5a-5c, it will be appreciated that the end pumped, single pass fiber optic amplifiers employed in fabricating the laser amplifier are constructed using lossy elements such as dichroic coupling elements to facilitate admission of the laser signal to be amplified and the pump beam at the same end of the optic fiber amplifier section.
What is needed is an optical fiber amplifier network allowing signals from many low power amplifiers to be coherently combined on a single optical fiber. Moreover, what is needed is an optical fiber amplifier network which minimizes the number of lossy elements employed in the network. Furthermore, an optical fiber amplifier network which can easily be scaled up to any required power level would be extremely desirable, particularly when the optical fiber amplifier network can be employed as a discrete module in a high-power laser amplifier. Lastly, an optical fiber amplifier network which mitigates problems with nonlinear parasitic effects that plague a single fiber amplifier of equivalent overall power would be particularly advantageous.